1. Field of the Invention
The present invention relates to a novel photoresin relief printing plate. More particularly, the present invention is concerned with a photoresin relief printing plate comprising a support and a relief portion formed thereon, which relief portion has a multilayer structure comprised of a soft upper layer and a hard lower layer, thereby providing the printing plate with not only excellent ink transferability of printing images having large solid areas, but also excellent image reproducibility.
2. Discussion of Related Art
Photoresin relief printing plates are generally produced by laminating, in the following order, one or more photosensitive resin layers and an image-bearing transparency on a support made of a synthetic resin, metal or paper which has an adhesive and an anti-halation layer thereon; irradiating the resultant laminate with active rays from the side of the image-bearing transparency to photocure selected areas of the photosensitive resin layer; and removing the unexposed, uncured areas of the irradiated laminate to form a relief layer.
On the other hand, an offset printing system is known to exhibit excellent printing performances, but is disadvantageously complicated in operation. Therefore, in the field of a relief printing system which is simple in operation, there is an increasing demand for developing a printing plate which not only has excellent printing resistance but also exhibits excellent printing performances, such as high resolution, excellent ink transferability and uniformity in printing images having a larger solid area and excellent tone reproducibility.
In relief printing, generally, the ink transferability and image resolution of a printing plate depend on the hardness of the printing plate. Illustratively stated, a relief printing plate having a high hardness is excellent in resolution but poor in ink transferability, whereas a relief printing plate having a low hardness is excellent in ink receptivity and ink transferability and therefore can provide a uniform print without dimness in printed images having a large solid area, but is susceptible to printing pressure and is likely to cause printed images to be broadened, rendering it difficult to attain high resolution.
In order to cope with the above-mentioned problems that are associated with conventional relief printing, an attempt was made to modify the photopolymerization characteristics of a photosensitive resin composition so as to improve the image resolution of the ultimate relief printing plate and the sharpness of the top edge portions of reliefs of the ultimate relief printing plate for rendering sharp the printed images (see Japanese Patent Application Publication Nos. 35-16614 and 37-1306). However, with respect to the relief printing plate of the above-mentioned type, when the hardness of the printing plate is high, a high printing pressure is needed to realize high ink transferability free from dimness in printed images having a large solid area. Therefore, although the printing plate has a high resolution, a marginal zone increase in the half-tone areas cannot be avoided due to the high printing pressure applied and consequently the quality of the obtained impressions is likely to be poor. Herein, the term "marginal zone" is intended to mean an unwanted printing area formed due to the ink protruding outwardly from the upper surface areas of the relief when the relief is pressed against a substrate to be printed. On the other hand, when the hardness of the printing plate is low, high ink transferability can be attained, but the broadening of the printed images, particularly at their small areas, disadvantageously becomes large. Therefore, the relief printing plate of this type is incapable of producing high grade impressions, such as those which are obtainable by offset printing.
Further, another type of photoresin relief printing plate was proposed, which has a relief comprising a surface layer and a base layer having a hardness which is smaller than that of the surface layer. This printing plate was devised for the purpose of decreasing the difference between the printing pressure exerted in the relief portions having small surface areas and the pressure exerted at the relief portions having large surface areas of the printing plate. The difference in printing pressure causes printed images at their small areas to be broadened disadvantageously. This proposed printing plate is effective for reducing such a pressure difference because the high printing pressure exerted at the small-area upper surfaces of relief portion is effectively absorbed by the soft base layer due to its cushioning action (see Japanese Patent Application Publication No. 61-56506). With this proposed photoresin relief printing plate having hard relief surface layers, however, when such a high ink transferability as can be attained by offset printing technique is intended to be obtained with respect to images having a large solid area by the use of a printing plate of this type, but of a wholly relatively hard type, it is necessary to apply a printing pressure higher than that for printing, by means of a printing plate having a single-layer relief having the same hardness throughout the entire thickness as that of the surface layer of this proposed printing plate, due to the cushioning action of the base layer. Therefore, in such a case, even if the broadening of printed lines constituting each half-tone image can be suppressed to some extent, it is impossible to attain a high ink transferability with respect to images having a large solid area and a high resolution which are both comparable to those as attained by offset printing technique. On the other hand, when a printing plate of this type, but of a wholly relatively soft type is used, a satisfactory ink transferability can be attained with respect to images having a large solid area, but the broadening of printed lines constituting each half-tone image becomes inevitably large so that a high resolution of images cannot be attained when a printing machine having poor machine accuracy is used or when printing is conducted on a substrate having poor surface smoothness.
Conventionally, ink transferability (and hence the quality of printed solid images) has been assessed by the determination of the reflection density (which is expressed by the common logarithm of the ratio of the luminance of a non-absorbing perfect diffuser to that of the surface under consideration, when both are illuminated at an angle of 45.degree. to the normal and the direction of measurement is perpendicular to the surface) of images. However, the present inventors have found that the quality of solid images is greatly affected by the visual coarseness of the images, which visual coarseness is derived from tiny vacant areas in the images, and hence ink transferability cannot be assessed simply by the determination of the reflection density and should also be assessed by the determination of the total area of the vacant areas in the images.
In these situations, the present inventors have devised a method for the quantitative evaluation of ink transferability by the determination of the ratio of the vacant areas in solid images (hereinafter often referred to simply as "vacant area ratio"). In this method, the vacant area ratio (%) is measured by using a half tone dots analyzer and is expressed in terms of the percentage of the total area of the vacant areas in the image at a predetermined given level of reflection density (this level is called "reflection density slice level"). For example, with respect to a solid image printed on a newspaper by offset printing, the vacant area ratio is 2% or less for high grade printing and even low grade printing exhibits a vacant area ratio of at most 5%, at a reflection density slice level of 0.5 D when the reflection density of a portion uniformly coated with the ink is taken as 1 D. In contrast, with respect to a solid image printed on a newspaper by relief printing using conventional printing plates, the vacant area ratio is 5% or so even for high grade printing and low grade printing exhibits a vacant area ratio as high as about 15%. The vacant area ratio of images produced by conventional relief printing techniques is generally 5 to 10%. That is, it is apparent that relief printing using conventional relief printing plates is largely inferior to offset printing with respect to ink transferability.
Thus, the vacant area ratio is a yardstick for properly assessing ink transferability.
In relief printing, a low printing pressure and a small amount of ink are usually employed in order to prevent a broadening of printed fine lines. Therefore, the vacant area ratio of a solid image printed by relief printing is likely to be disadvantageously high. In contrast, impressions obtained by offset printing generally has excellent ink transferability, that is, it exhibits a vacant area ratio as low as 5% or less, preferably 3% or less. In the field of relief printing also, it has been desired to develop a relief printing plate exhibiting a vacant area ratio of the level attained by offset printing.
On the other hand, with respect to the broadening of printed lines, in the field of printing which requires high precision, such as newspaper printing, excellent sharpness of impressions can be obtained when the broadening in line width as from a relief line having a width of 60 .mu.m to a printed line is generally 60 .mu.m or less. In the field of printing which does not require so high a precision, such as corrugated board printing, good sharpness of impressions can be obtained when the broadening in line width as from a relief line having a width of 300 .mu.m to a printed line is generally 200 .mu.m or less.
However, with respect to the conventional relief printing plates, when the vacant area ratio in printed solid images is reduced, the broadening of printed lines inevitably becomes large, and vice versa. There is a serious technical dilemma to be solved.